Consider this: A medical device that people's lives depend on. It only fails in 1/1000 cases causing death. Many people could state, "it works for me, can't be broken!" On the other hand, the families of the dead could argue that it is broken. Who is right?
Obviously this isn't such an extreme case. No one has their life depending on a Vim plugin, but it illustrates a point. "Works for me" doesn't necessarily imply "isn't broken."
I think the point is that for many people building free software for fun, "works for me" is all that matters. Testing use cases that you know you will never encounter is not interesting or challenging (at least in this case), but it takes time, and you're not making a product, nobody is relying on you. Why bother?
The author of this plugin isn't trying to make a spreadsheet competitor, they just released it publicly because other people might find it useful or interesting.
We can consider it incomplete, or disagree with the design choices, but broken means it doesn't do what it's supposed to do. Here, it does everything the author intended it to do, and everything the description says it does. When you say it's broken, it sounds at least to me like you're imposing your own requirements on a project you have nothing to do with. It's like calling Microsoft Office broken because it can't handle Open Office files. It's not broken, it just doesn't have all the features you would have included.
